PROJECT SUMMARY Peri-implant bone loss, mostly due to peri-implantitis after implants are in function, can cause implant loss if not diagnosed and treated in a timely manner. It is estimated that peri-implantitis affects approximately 20% of implants. Tissue loss and compromised function negatively impact the welfare of hundreds of thousands of patients. Currently peri-implant bone loss is primarily assessed by 2-dimensional (2D) periapical radiographs. The main limitation, i.e. only providing superimposed images, inevitably reduces its diagnostic value. Cone- beam computed tomography provides useful cross-sectional imaging but is recommended not to be used for periodic post-op assessment. Non-ionizing and point-of-care ultrasonography can also offer cross-sectional images and holds great potential to complement radiographic uses. Our group, among others, has shown accuracy of ultrasound to measure facial alveolar bone crest dimensions. Therefore, ultrasound can potentially augment clinicians' ability to assess peri-implant bone level, especially on the facial and palatal/lingual sides. Additionally, marginal bone thickness, another important clinical parameter for assessing long-term peri- implant soft and hard tissue stability, can be measured with ultrasound. The primary aim of this proposal is to validate the accuracy of 3D ultrasonography for measuring peri-implant marginal bone dimensions. Three aims are proposed to achieve this objective: Aim 1: Establish optimal imaging settings for the ultrasound system for peri-implant marginal bone measurements. These settings include choice of transmit frequency, harmonic and compound use. Hypothesis 1: Ultrasonic imaging can delineate oral hard tissue (bone) edges for use in diagnostic imaging procedures (with <10% error, i.e. the same error as the current standard method 2D radiographs) and provide dynamic range for differentiation of tissue types by relative contrast. Aim 2: Establish intra- and inter-rater reliability by conducting a calibration exercise for ultrasound bone measurements in humans. Hypothesis 2: Intra-class correlation coefficient (ICC) within and between examiners can reach at least 0.85. Aim 3: Compare ultrasound measurements to intraoperative direct and radiographic measurements in a cross- sectional human investigation. Hypothesis 3: Mean ultrasonic bone level and thickness measures differ <0.5 mm and <0.3 mm, compared to the corresponding open bone intraoperative bone measurements, respectively. Ultrasound measures would be non-significantly different from radiographic measures. Successful completion of this proposal will validate non-invasive, real-time and high-resolution ultrasonography for assessing peri-implant marginal bone dimensions. In future investigations we will test ultrasonography for characterizing peri-implant bone loss and its progression. We will also measure treatment efficacy in larger scale human studies using ultrasonography.